Plate heat exchanger

ABSTRACT

Plate heat exchanger consisting of a plate stack ( 2 ) and a housing ( 1 ) surrounding the plate stack, wherein the plates ( 2   a ) are connected with one another in the plate stack ( 2 ) in such a manner that a first and at least one further medium can flow alternately through adjacent plate interstices, and the one medium can be conducted to and away from the plate stack ( 2 ) by way of through-flow channels that run in the stack direction, while the other medium can be conducted to and away from the plate interstices assigned to it by way of an interstice between housing ( 1 ) and plate stack ( 2 ) on the outer circumference. It is essential, in this connection, that the following combination of materials is present: The plates ( 2   a ) of the plate stack ( 2 ) consist, in known manner, of corrosion-resistant material, whereas the housing ( 1 ) consists of a non-corrosion-resistant material that has an enamel coating on its inner side, which is impacted by medium.

BACKGROUND

The invention relates to a plate heat exchanger consisting of a plate stack and a housing surrounding the plate stack, wherein the plates are connected with one another in the plate stack in such a manner that a first and at least one further medium can flow alternately through adjacent plate interstices, wherein the one medium can be conducted to and away from the plate stack by way of through-flow channels that run in the stack direction, while the other medium can be conducted to and away from the plate interstices assigned to it by way of an interstice between housing and plate stack on the outer circumference.

Plate heat exchangers of this type are known in numerous embodiments. The present invention relates not only to those constructions in which the plates have seals at their edge and around part of their through-flow openings and are tightly pressed together in the stack direction. This construction has the advantage that the plate stack can be opened, inspected, and cleaned in simple manner.

In addition, however, the present invention also relates to fully welded plate heat exchangers in accordance with U.S. Pat. No. 6,158,238, US 2005/0039896, WO 2008/046952, and WO 2010/149858, the content of which is also incorporated into the present application in order to avoid unnecessary repetition. In this connection, the plate stack consists of a plurality of plate pairs, where each plate pair is formed by two plates that are welded to one another at least along their outer circumference. Each plate has at least two openings to allow a medium to flow through, adjacent plate pairs being connected with one another along these openings by means of welding, so that the said medium gets from one plate pair to the next through these openings. In contrast, the other medium flows through the interstice between housing and plate stack, and gets into the interstices between adjacent plate pairs by way of the outer circumference of the plate stack.

Furthermore, the invention also relates to mixed forms of the two aforementioned plate heat exchanger constructions, in other words to partly welded heat exchangers, in which plate pairs welded to one another are pressed against one another by way of seals, in other words only the interstices between adjacent plate pairs can be opened at any one time.

Of course, these plate heat exchangers can also be configured so that more than two media participate in the heat transfer or so that heat-transferring plate interstices do not follow one another directly, but rather an inactive plate interstice is placed in between, which functions as a safety cushion, for example.

To the extent that it was discussed initially that adjacent plate interstices alternately have a first and at least one further medium flowing through them, this therefore does not mean that this must always involve directly adjacent interstices; instead, a third medium can be accommodated in the directly adjacent interstice, whether as a stationary medium or as a through-flowing medium.

With regard to the material for the plate heat exchangers, it is usual to use stainless steels that are alloyed with chromium and nickel. For corrosion resistance to acids, for example, it is furthermore known to add molybdenum as a further alloy component, or to use plates composed of titanium, nickel or their alloys. However, these materials have the disadvantage that they are very cost-intensive. It has therefore become known from WO 2008/046952 to produce the plates and the housing of the heat exchanger from carbon steel with a carbon content of 0.05% to 2.1%, from which sufficient corrosion resistance is supposed to result for many application cases.

SUMMARY

The present invention is based on the task of improving heat exchangers of the constructions described initially, to the effect that on the one hand, excellent corrosion resistance is guaranteed, and on the other hand, the production costs are significantly more advantageous than before.

This task is accomplished, according to the invention, in that the following combination of materials is selected: The plates of the plate stack consist, in known manner, of corrosion-resistant material, particularly stainless steel, whereas the housing consists of a non-corrosion-resistant material, which has an enamel coating on its inner side, which is impacted by medium.

By means of this combination of materials, the material costs of the housing are significantly reduced, without detriment to its corrosion resistance. Therefore great corrosion resistance is achieved at lower costs.

Fundamentally, the housing of the heat exchanger can consist of almost any desired material, as long as it is sufficiently stable and firm and suitable as a support for the enamel coating. It is particularly practical to produce the housing from enameled black steel.

It is practical if a multi-layer enamel coating is used for the enamel coating consisting of a base enamel that is characterized by good adhesion to the metallic support material of the housing and by smoothing of the same, and of a cover enamel that demonstrates great chemical resistance, and, in particular, is acid-resistant.

For the material composition of the enamel coating, at least with regard to its cover layer, it is recommended that it contains more than 50%, preferably more than 60% SiO₂ and/or more than 3.5%, preferably more than 4.5% TiO₂ and/or more than 12%, preferably more than 15% Na₂O.

Furthermore, it is recommended that the enamel coating, at least in its cover layer, contains B₂O₃ and/or Ka₂O and/or Li₂O and/or MoO₃ and/or MnO and/or ZrO₂ and/or F, preferably in the one-digit percentage range, in each instance. In addition, it is recommended to add BaO and/or CoO and/or V₂O₅, preferably in the parts per thousand range, in each instance.

The thickness of the enamel coating amounts to about 1 mm to 3 mm, preferably about 2 mm.

For the connector pieces usually welded onto the housing, a person skilled in the art is aware of known alternatives, particularly, therefore, of the use of stainless steel. However, it also lies within the scope of the invention to use enameled black steel also for these connector pieces, particularly to the extent that they are provided for the medium flowing through the interstice between housing and plate package.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention are evident from the following description of an exemplary embodiment and from the drawing; in this connection, the figures show:

FIG. 1: a section through the plate heat exchanger according to the invention, transverse to the plates;

FIG. 2: a top view of the outside and the inside of a plate pair;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawing, a housing 1 can be seen, which surrounds a plate package 2 in known manner. The housing as well as the plate package can have a rectangular shape, but also a circular or other contour.

The plate package 2 consists of plates 2 a welded to one another in pairs, each plate pair having a medium flowing through it, which is conducted in by way of at least one connector piece 3 in the upper region of the plate package and conducted away by way of at least one connector piece 4 on the lower region of the plate package. For this purpose, the plates 2 a have through-flow openings, in known manner, which align with the connector pieces 3 and 4.

Adjacent plate pairs are welded to one another at least along these through-flow openings, so that through-flow gaps for the other medium are formed between all the adjacent plate pairs. This other medium is conducted into the upper circumference of the plate package 2 by way of a connector piece 5, leaves the plate package at the lower edge, and is conducted away by means of a connector piece 6. In this connection, known installations la in the housing 1 ensure that the medium last mentioned cannot flow around the plate package on the outside, but rather must flow through the gap between the plate pairs.

The heat exchanger described is structured to be fully welded and without seals. Depending on the flow direction selected, it can be operated in co-flow, counter-flow, or cross-flow mode.

It is now essential that the housing 1 consists of non-corrosion-resistant material and has a technical enamel coating on all its inside walls—at least to the extent that they are impacted by a related medium—having a thickness of about 2 mm. In contrast, the plate package 2 and its connector pieces 3 and 4 consist of corrosion-resistant stainless steel.

To the extent that the inner coating of the housing 1 with enamel was mentioned above, of course this also relates to the installations 1 a that serve for conducting the flow, which are disposed not only on the face sides of the plate stack 2, but rather also in the circumference regions of the plate stack.

For the configuration of the housing 1, it is recommended that it can be opened from at least one face side, in that a releasable lid is provided there. Of course, this lid can also have the enamel coating described above on its side impacted by medium. If direct contact of opposite enamel layers occurs when the lid is screwed in place, this is preferably avoided in that an elastic seal composed of corrosion-resistant plastic is laid in between.

A particularly practical composition of the enamel coating, at least on its outer cover layer, can be defined as follows:

-   -   a. 65% SO₂, 15% Na₂O, 5% TiO₂, 3.5 ZrO₂, 2.5% B₂O₃,     -   b. 1.8% F, 1.7% Li₂O, 1.4% MnO, 1.1% MoO₃, 1% Ka₂O,     -   c. 0.3% BaO, 0.25% V₂O₅, and 0.1% CoO;

in this connection, it does, of course, lie within the scope of the invention to change the aforementioned percentages upward or downward from the initial value by 5% to 10%, in each instance.

FIG. 2 illustrates the possibilities as to how adjacent plate pairs and how the plates of a plate pair are connected with one another or can be releasably attached to one another, the former being shown in the left half, the other view in the right half of FIG. 2.

By analogy to the reference symbols used for the connector pieces in FIG. 1, the through-flow opening in the plate 2 a assigned to the upper connector piece 3 is marked with the reference symbol 3 a, and the through-flow opening assigned to the lower connector piece 4 is marked with the reference symbol 4 a.

In the top view of the outside of a plate pair shown on the left, the connection with the adjacent plate pair—whether it is releasable or non-releasable—takes place along the circumference of the through-flow openings 3 a and 4 a, in other words in the blackened region 13 a or 14 a. In these regions, which in practice one should imagine to have not the circumference ranges of 180° shown in the drawing, but rather the full circumference ranges of 360°, adjacent plate pairs can be welded to one another.

Likewise, it is possible to provide a releasable connection in place of a welded connection; then, a seal is disposed in the blackened regions, of course once again over the entire opening circumference, and the plate pairs are pressed together axially, from the outside, so that adjacent plate pairs follow one another, in sealed manner, in the circumference region of the through-flow openings 3 a and 4 a.

In both cases, the interstice between adjacent plate pairs that lies outside of the weld seams or seals 13 a and 14 a has the other medium flowing through it, which medium is conducted in or away by way of the connector pieces 5 and 6.

In the right half of FIG. 2, the connection of two plates of the same plate pair is shown, specifically with a view from the interior of the plate pair. Here, the plates are welded to one another along their outer circumference, in each instance, or releasably pressed against an intermediate seal. This welded or sealed region is marked with the reference symbol 15, where here, too, it must be kept in mind that the welded or sealed region extends over the entire circumference, in other words over 360°.

The through-flow openings 3 a and 4 a are open toward the interior of the plate pair, so that the medium can flow from the through-flow opening 3 a through the interior of the plate pair to the through-flow opening 4 a.

The heat exchanger is shown only schematically in FIGS. 1 and 2, because in the present case, the only matter of concern is the special combination of materials, and the flow schematics and the welding variants of the plates correspond to the known state of the art.

In the end result, it can be stated that the present invention is characterized by excellent corrosion resistance at comparatively low production costs, by means of the material combination presented. 

1. A plate heat exchanger comprising a plate stack (2) of plates (2 a) and a housing (1) surrounding the plate stack, the plates (2 a) are connected with one another in the plate stack (2) in such a manner that a first and at least one further medium can flow alternately through adjacent plate interstices, and the first medium is conducted to and away from the plate stack (2) of through-flow channels that run in a stack direction, while the further medium is conducted to and away from the plate interstices assigned to it by way of an interstice between housing (1) and plate stack (2) on an outer circumference, the plates (2 a) of the plate stack (2) are formed of corrosion-resistant material, and the housing (1) is formed of a non-corrosion-resistant material that has an enamel coating on an inner side thereof, which is impacted by the medium.
 2. The heat exchanger according to claim 1, wherein the housing is formed of enameled black steel.
 3. The heat exchanger according to claim 1, wherein the enamel coating has multiple layers.
 4. The heat exchanger according to claim 1, wherein the enamel coating includes at least one base coating and at least one cover coating.
 5. The heat exchanger according to claim 1, wherein the enamel coating, at least in a cover layer thereof, contains more than 50% SiO₂.
 6. The heat exchanger according to claim 1, wherein the enamel coating, at least in a cover layer thereof, contains more than 3.5% TiO₂.
 7. The heat exchanger according to claim 1, wherein the enamel coating, at least in a cover layer thereof, contains more than 12% Na₂O.
 8. The heat exchanger according to claim 1, wherein the enamel coating, at least in a cover layer thereof, contains at least one of: B₂O₃, Ka₂O, Li₂O, MoO₃, MnO, ZrO₂, or F, in a single digit percentage range, in each instance.
 9. The heat exchanger according to claim 1, wherein the enamel coating, at least in a cover layer thereof, contains BaO, CoO, or V₂O₅, in a parts per thousand range in each instance.
 10. The heat exchanger according to claim 1, wherein the enamel coating has a thickness of about 1 mm to about 3 mm.
 11. The heat exchanger according to claim 1, wherein the housing (1) has welded-on installations (1 a) for guiding the flow, which also are formed of enameled black steel.
 12. The heat exchanger according to claim 1, wherein the housing (1) has welded-on connector pieces (5, 6), which also are formed of enameled black steel.
 13. The heat exchanger according to claim 1, wherein the housing (1) has housing parts (1 a, 1 b) that are screwable onto one another. 